High-frequency apparatus



April 15, 1952 P. L. SPENCER HIGH-FREQUENCY APPARATUS Filed Feb. 13, 1947 5y Ms Patented Apr. 15, 1952 HIGH-FREQUENCY APPARATUS Percy L. Spencer, West Newton, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application February 13, 1947, Serial No. 728,261

(Cl. 2l9-47) 3 Claims.

This invention relates to high-frequency apparatus, and more particularly to a microwave oven.

A microwave oven of the type to which this invention relates consists, generally, of a hollow metallic enclosure or cavity to the interior of which a source of radio-frequency energy, having a wavelength which may lie in the microwave region of the frequency spectrum, is coupled so as to supply radio-frequency or microwave energy thereto for the purpose of heating an object therein.

Often it is desired to increase the radio-frequency power supplied to the oven; this may be conveniently accomplished by coupling the output of a second radio-frequency oscillator to said oven. Also, in circuits in which the radiofrequency oscillator is connected for A. C'. operation in a self-rectifying circuit, for better vbalancing of, and for more eicient utilization of, the input transformer, it is desirable to connect a second radio-frequency oscillator to said transformer. However, it has been found that certain diiculties arise when two or more closely similar sources of: radio-frequency energy, for example devices of the so-called magnetron oscillator type, are coupled, by means oi radio-frequency transmission lines or waveguides, to the same cavity or enclosure. These diiiiculties arise from the fact that, when such devices have exactly the same output frequency, one of the devices tends to act as a low impedance across the output of the otherdevice, due to slight dissimilarities between them, so that one of the magnetrons tends to absorb the radio-frequency power of the other one, acting as an additional load across said other, instead of supplying radio-frequency power to the common load in the oven as desired. In other words, one of the magnetrons presents a rather low'impedance to the other. favorable condition is most noticeable, of course, when the two magnetrons are being operated on direct current, in which case they are both energized continuously; however, this condition tends to exist even during alternating current operation of the magnetrons, when they are energized only on successive half-cycles of the energizing source. since in this situation each magnetron may tend vto supply radio-frequency energy to the other during the half-cycle in which the first is energized rather than to the load in the oven.

An object of this invention, therefore, is to devise a radio-frequency enclosure supplied by a plurality of separate sources of radio-,frequency energy in an efficient manner,

Another object is to devise a method for coupling a pair of magnetrons to a single radiofrequency cavity in such a way that there is no appreciable interflow of radio-frequency energy between them.

A further object is to devise an effective means for supplying high radio-frequency power to a microwave heating apparatus.

The foregoing and other objects of the invention will be best understood from the following description of an exemplication thereof, reference being had to the accompanying drawing, wherein:

Fig. l is a vertical section through an oven according to my invention, together with a diagrammatic representation of a system for feeding radio-frequency energy to the same Fig. 2 is a partial horizontal section taken along line 2 2 of Fig. 1 and looking in the direction of the arrows; and

Fig. 3 is a partial vertical section of the oven taken along line 3-3 of Fig. l.

Now referring to the drawing, a hollow rectangular prismoidal enclosure or cavity l is made of a suitable metal and has rather thin walls as shown; enclosure l is adapted to serve as the oven of the heating or cooking apparatus. A container 2 of food or any other material to be heated may be placed inside oven I, said container resting on the bottom wall of the oven or being placed in any other suitable position therein while the food is being cooked. A metal door 3 is hingedly mounted on the front wall of the oven, this door being adapted to close an access aperture or opening provided in the front metallic wall of the oven. When door 3 is closed, the enclosure l is entirely closed, except for a pair of similar openings 4 and 5 in the opposite side walls of said enclosure; these openings are further described hereinafter.

Numeral 6 generally designates an electrondischarge device of the magnetron type, which includes, for example. an evacuated envelope i, made of highly conductive material, such as copper, and provided with a plurality of inwardly-directed, radially-disposed anode vanes 8. The arrangement is such that each pair of adjacent anode vanes forms, together with that portion of the envelope lying therebetween, a cavity resonator whose natural resonant frequency is, as is well known to those skilled in the art, a function of the geometry of the physical elements making up the same. For the purposes of the present invention it is Adesirable that the dimensions of each such cavity resonator be such that the Wavelength of the electrical oscillations adapted to be generated therein, or the t output frequency of the magnetron, has a predetermined value. For example, said wavelength may lie in the microwave region of the frequency spectrum, which region includes wavelengths less than about thirty centimeters.

Centrally located in envelope 'l is a highly electron-emissive cathode member Q, for example of the well-known alkaline-earth metal-oxide type, l

electron path between the cathode and anode members thereof.

Magnetron 6 is energized from a suitable source, in a manner to be described hereinafter, and when so energized delivers radio-frequency electromagnetic energy of a predetermined wavelength to a hollow rectangular waveguide it through a coaxial transmission line II which is coupled to magnetron oscillator 5 by loop I2. The central conductor I3 of line Il extends into the interior of waveguide Iii near one end thereof, through an opening provided in the center of the longer side of said guide, to serve as anv exciting rod or exciting probe for said guide.

The end of guide lil nearest this exciting'v rod is closed, while the opposite end of said guide is fastened to the left side wall of enclosure I by fastening means Ifl and is open. The interior of guide lil is placed in energy-transmit'- ting relationship with the interior of the cavity l by means of a rectangular aperture l provided in the left side wall of said cavity, this aperture being of the same size and vconguration as the interior of guide I and being aligned with said guide to place the interior of said guide in communication with the interior of cavity I. Aperture d is located in a central position with respect to the front and rear walls of oven l, as shown in Fig. 2, and is preferably located above thehorizontal midplane4 of the oven.

By means of the above-described structure, the radio-frequency output of source or oscillator 6 is coupled to the interior of enclosure I. The exciting rod of guide It is arranged to set up TEM mode waves therein, as will be apparent to those skilled in the art. These waves propagate down the waveguide and emanate from the open end thereof into the cavity I.

As stated above, for the purposes of increasing the radio-frequency power supplied to oven I, a second radio-frequency oscillator I6 may be utilized, the radio-frequency output of this second oscillator also being coupled to oven I. Oscillator I6 is of the same type as oscillator t and includes an outer envelope I'I, anode vanes I8, and a cathode member I9.

The cathodes 9 and I9 of the electron-discharge devices and I6 are connected, respectively, by conductors I and 25, to the opposite terminals of the secondary winding 28 of an input transformer 29, the primary winding 30 of said transformer being connected to a source of raw A. C., for example, the conventional 60- cycle power lines. The conductive envelopes 'I and I'I of said electron-discharge devices 6 and I6 are tied together by conductors 3| and 26, which are, in turn, connected, by a conductor 4 21, to a center tap on the secondary winding 28 of the transformer 29.

The radio-frequency output of oscillator I, which has a predetermined wavelength, is coupled to enclosure I by means of a coaxial transmission line 2| and a hollow rectangular waveguide'20, line 2| being coupled to magnetron oscillator I6 by loop 22. Central conductor 23 of line 2| extends into the interior of waveguide 20 to serve as an exciting rod or exciting probe therefor. The open end of guide 25 is fastened to the right side wall of oven by fastening means 24, and the interior of said guide is placed in. energy-transmitting relationship with the interior of cavity I by means of a rectangular aperture 5 aligned with said guide, aperture 5 being similar to aperture and being located in the right side wall of enclosure I directly opposite aperture li.

With the two magnetrons connected in a circuit as above described, and with the radiofrequency output of both sources coupled to the interior of the enclosure, the electron-discharge devices o and I6 are connected for push-pull A. C. operation in a self-rectifying circuit, whereby said devices alternately deliver hyperfrequency energy or microwave energy to the common enclosure or cavity I.

I have found that, if the predetermined nat ural resonant frequencies of oscillators 6 and I6 are exactly the same, or in other words, if the two oscillators have exactly the same output frequency, one of the devices tends to act as a low impedance across the output of the other, so that one of the devices tends to absorb radiofrequency energy from the other, which absorption is made possible by the radio-frequency coupling of both to the same enclosure. However, I have discovered that if the predetermined radio-frequency output wavelengths 'of the two devices 6 and I6 differ apprecably, this tendency of one of the magnetrons to absorb radio frequency energy or radio frequency power from the other may be reduced to a level that is inappreciable. As a result, an inappreciable amount of such power is shunted away from the load. I have found that when the predetermined wavelengths of the two magnetron oscillators are different from each other as above explained, each magnetron will pre'- sent to the other an impedance which is suffi'- ciently high to prevent any appreciable absorption of radio-frequency energy by either oscillator from the other,l or to prevent any appreciable transfer of radio-frequency power from one magnetron to the other. Expressed in 'another way, when the output frequencies of the two magnetron oscillators are sufficiently different from each other, the impedances of the two oscillators are also sufficiently different from each other to prevent any appreciable transfer of radio-frequency pcwer'from one to the other.

As an example of the output frequency differ'- ence above alluded to, I have found that at frequencies on the vorder of 2450 megacycles (frequencies on this order lie within the microwave band), there should be sa frequency difference on the order of 25 to 50 megacycles between the two magnetrons, in order for two magnetrons or radio-frequency oscillators to operate eiii'- ciently, according to this invention, when coupled to the same metallic cavity or enclosure.

It 'will be noted from all ofthe foregoing lthat I have devised a simple yet effective means for coupling a plurality 'of sources of radio-frequency energy, such as magnetron oscillators, to a single radio-frequency cavity, in such a way that there is no appreciable interfiow of radio-frequency energy between them.

Of course, it is to be understood that this invention is not limited to the particular details as described above, as many equivalents Will suggest themselves to those skilled in the art. For example, when the two magnetrons are being operated on alternating current, it is possible to use a single or common waveguide for coupling the radio-frequency output of the two magnetrons to the oven. Also, although in Fig. 1

` alternating current operation of the magnetrons is disclosed, my invention is equally applicable t0 direct current operation of the magnetrons. In fact, as stated above, the diiculties existing, when two equal-frequency magnetrons are operating on direct current, are even greater than when the magnetrons are operating on alternating current. Various other variations will suggest themselves. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of this invention within the art.

What is claimed is:

1. Heating apparatus, comprising a metallic enclosure in which a dielectric body may be placed for heating the same, a plurality of magnetron oscillators each having a different `output frequency, whereby said oscillators present each t0 the others an impedance sufliciently high to prevent appreciable absorption of electromagnetic wave energy by any oscillator from the others, and means for coupling the wave output of said oscillators to the interior of said enclosure to heat said body, said enclosure being substantially nonresonant at the operating frequencies of said oscillators.

2. Heating apparatus, comprising a metallic enclosure in which a dielectric body may be placed for heating the same, a plurality of magnetron oscillators each having a different output frequency, whereby said oscillators present each to the others an impedance sufficiently high to prevent appreciable absorption of electromagnetic wave energy by any oscillator from the others, and means for coupling the wave output of said oscillators to the interior of said enclosure to heat said body, said enclosure substantially preventing electromagnetic wave energy transfer between the interior and exterior thereof except between said interior and said oscillators, said enclosure being substantially non-resonant at the operating frequencies of said oscillators.

3. Heating apparatus, comprising a metallic enclosure in which a dielectric body may be placed for heating the same, means for supporting said dielectric body said enclosure, a plurality of magnetron oscillators each having a different output frequency, whereby said oscillators present each to the others an impedance sulciently high to prevent appreciable absorption of electromagnetic Wave energy by any oscillator from the others, and means for coupling the wave output of said oscillators to the interior of said enclosure to heat said body, said enclosure being substantially non-resonant at the operating frequencies of said oscillators.

PERCY L. SPENCER.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 2,250,308 Lindenblad July 22, 1941 2,337,184 Carter Dec. 21, 1943 2,341,120 Rudd et al Feb. 8, 1944 2,381,323 Vore Aug. '7, 1945 2,402,397 Hansell June 18, 1946 2,474,938 Gorn July 5, 1949 2,495,429 vSpencer Jran. 24, 1950 

